Preparation And Properties Of Vanadium/Zirconium Conversion Film On AZ31 Magnesium Alloy

Magnesium alloys are widely used in transportation,information and communication,mechanical manufacturing and other hot fields because of their excellent metal properties.However,due to the unstable chemical properties of magnesium alloys,they are easily eroded by the surrounding atmospheric environment.Corrosion resistance protection of magnesium alloys has become a practical problem to be solved urgently.Chemical transformation treatment technology has the characteristics of high efficiency and simple operation.Nowadays,green and environmentally friendly new chemical conversion technology has become a hot research direction.In this paper,non-toxic and environmentally friendly NaVO₃ and K₂ZrF₆ were selected as the main salt for film formation,SDBS was selected as the auxiliary component,and the corrosion resistance,adhesion and thickness of the conversion film were investigated based on the evaluation criteria of drip test,scratch test and thickness test.The composition and process parameters of the conversion solution were optimized by single factor and orthogonal experiments.At the same time,the morphology,composition and electrochemical depth of the conversion film were investigated by using relevant testing equipment.The properties of conversion membranes and the mechanism of film formation were discussed.The transformation schemes were optimized by single factor experiment and orthogonal experiment.The conversion schemes were NaVO₃ 3.5g/L,K₂ZrF₆ 3.5g/L,SDBS 2.8g/L,conversion time 25min,conversion temperature 50℃and conversion solution pH 2.6.The results of metallographic microscopy,scanning electron microscopy（SEM）and energy dispersive spectroscopy（EDS）analysis show that the conversion film under the optimal scheme is uniform and compact,without obvious holes and cracks.V,Zr,F,O,Mg and C are the main film forming elements,and V and Zr are the main film forming elements.X-ray photoelectron spectroscopy（XPS）analysis showed that the actual substances constituting the film were ZrO₂,V₂O₅,MgO and a small amount of ZrF₄,MgF₂.Straight-chain sodium dodecyl benzene sulfonate could not only adsorb on the surface of the film,but also increase the expansion and permeability of the conversion solution on the surface of magnesium alloy.The protective effect of magnesium alloy matrix is improved.The electrochemical analysis of conversion coating by electrochemical workstation shows that the electrochemical properties of magnesium alloy specimens with conversion components are better than those of bare magnesium alloy matrix.Under the optimal scheme,the corrosion resistance current of the conversion film is 0.102×10^-5A/cm² and the corrosion resistance potential is-1.438V.The AC impedance analysis results show that the conversion film plays a key role in blocking the exchange of ions between the internal and external ions of the matrix,and the protective performance of the conversion film is the best.